Well, mine do that when I first start driving it after its had time to cool off. Only happens in the winter and especially when the brakes are moist. I think its the pads and rotors being cold/ wet. When you first hit the brakes it doesnt quite grab it and then it locks them up cuz it all of a sudden gets good friction. Normal for me to have that happen for the past 3 years that ive payed attention.